This invention relates to plasticating resin using a screw rotating in an extrusion or injection molding barrel, from which the resin extrudes or flows to form a product in a die or mold. More particularly, the invention pertains to the arrangement and structural form of various sections of a screw that is especially suited for use in plasticating polypropylene resin.
Polypropylene has been commercially available since 1957 and has remained the fastest growing major thermoplastic, its growth rate being greater than that of many other commodity polymers. The resin is readily available at relatively low cost, and it possesses many unique properties that are suitable for a wide variety of consumer and industrial products. Its excellent crack resistance, low creep rate, low density and high melting point have contributed to its broad applications in injection molding, film, fiber and filaments.
Its fabrication, however, requires an extrusion step in which resin pellets are converted to a molten state to form a desired product. It is well known that polypropylene is more difficult to extrude than polyethylene. Generally, polypropylene output from an extruder of a given size is lower than the output of polyethylene because polypropylene resins plasticate in single-screw extruders slower than do polyethylene resins. When processing polypropylene resin on a conventional screw specifically designed for processing polyethylene, the output rate reduction for polypropylene is about 30 percent compared to polyethylene.
The process variables and equipment for extruding polypropylene have received much attention due to its low output rate, low-melt strength, and sensitivity to shear. The chemical and physical properties that contribute to the extrusion difficulties of polypropylene are its high crystallinity and stiffness. High crystallinity requires greater heat input at the melting section of an extruder in order to supplement the energy required for melting. Its low melt viscosity reduces the effectiveness of viscous shear heating in the melting section; therefore, its rate of melting is lower than that of polymers having a higher melt viscosity. High stiffness or rigidity makes it more difficult for polypropylene resin pellets to be compressed and transported in the feed and compression sections of a plasticating screw.
In order to improve the extrusion throughput rate of polypropylene, attempts have been made to improve the onset of melting and the rate of melting by increasing the magnitude and rate of external heat input through the barrel wall to the resin. To improve the throughput rate of polypropylene, some extruder designers choose a barrel diameter that is one size larger than would be required to produce the same output rate with low-density polyethylene resin. Data produced with plasticating apparatus including a screw having a feed section, a transition section and a metering section, but no barrier section, were reported by E. E. Strangland et al. and by C. Y. Cheng in separate technical papers that contain data from plasticating polypropylene.
But a smaller diameter extruder has a larger heated surface area relative to its output rate. Therefore, a small extruder would permit greater heat input at the melting zone to supplement energy required for melting than would a larger extruder. Data for polypropylene processed with a small diameter extruder was reported by C. Y. Cheng in a technical paper published in 1988.
At best these conflicting design choices merely compensate for the physical and chemical properties of polypropylene that diminish its output rate, and they potentially impose higher equipment costs and increase energy costs. Despite these compromises, there remains a need to improve the extrusion output rate of polypropylene using a process and apparatus that do not unnecessarily increase production costs.